If you if you don't have salespeople then you need to make a product that works and fulfills user needs. And it has to be good enough for word of mouth...which is where posthog's experience comes in.
It's not precisely so. They do and have had partners that did sales / consulting work.
In my limited experience partners are usually small integrators that do maybe a small urban area. Did they get a big consulting partner/implementor or something?
Those do exists. Partner was a gray area for them. Some services like certain support was also outsourced to partners. In a way the partners grew with the company so at some point (when it peaked) there were a lot of "large" partners.
That's why I said it is confusing. There was a time when you contact them on the website looking for a solution someone would redirect you to a partner. It was essentially outsourced sales / solution team(s).